When grouping is required, it is better to use this solution (only SQL2012 and later)

select
cte.EPICSystem_Title
,cte.Uren
,cte.Realisation
,cte.Datum
,cte.Budget
,sum(cte.uren) over (partition by cte.EPICSystem_Title order by datum) as CumUren
,sum(cte.Realisation) over (partition by cte.EPICSystem_Title order by datum) as CumRealisation
from cte